1. Field of the Invention
The present invention is directed to shale shakers with vibrating screens, and, in one aspect, to shale shakers with tiered or tandem screens.
2. Description of Related Art
The prior art discloses a wide variety of vibrating screens, devices which use them, shale shakers, and screens for shale shakers. In shale shakers which use a plurality of screens, problems arise when an initial screen encounters such a large amount of fluid, e.g. drilling mud, that adequate and efficient screening is not possible.
The need for solids control in drilling mud in hydrocarbon well drilling is well known in the prior art. Drilling mud, typically a mixture of clay, water and various additives, is pumped through a hollow drill string (pipe, drill collar, bit, etc.) down into a well and exits through holes in a drill bit. The mud picks up cuttings (rock bits) and other solids from the well and carries them upwardly away from the bit and out of the well in a space between the well walls and the drill string. At the top of the well, the solids-laden mud is introduced to a shale shaker, a device which typically has a series of screens arranged in tiered or flat disposition with respect to each other. The screens catch and remove solids from the mud as the mud passes through them. If drilled solids are not removed from the mud used during the drilling operation, recirculation of the drilled solids can create viscosity and gel problems in the mud, as well as increasing wear in mud pumps and other mechanical equipment used for drilling. In some shale shakers, a fine screen cloth is used with the vibrating screen. The screen may have two or more overlying layers of screen cloth. The frame of the vibrating screen is suspended or mounted upon a support and is caused to vibrate by a vibrating mechanism, e.g. an unbalanced weight on a rotating shaft connected to the frame. Each screen may be vibrated by vibratory equipment to create a flow of trapped solids toward an end of the screen on a top surface of the screen for removal and disposal of solids. The fineness or coarseness of the mesh of a screen may vary depending upon mud flow rate and the size of the solids to be removed.
In certain prior art shale shakers having one (or more) processing screens, such screens cannot adequately deal with a surge in fluid flow or high fluid flow rates, e.g. During a xe2x80x9cbottoms upxe2x80x9d or riser pipe circulation condition. La other prior art systems, the discharge of one or more shale shakers is fed to another shale shaker for further de-liquefying and de-oiling. Such a process necessarily requires at least two shale shakers. However, on offshore drilling rigs, space is at a premium. For this reason, tiered or tandem shale shakers are used to effect a finer screening of the mud on the second level. But, in high volume operations, such tandem shale shakers may not be able to handle the throughput due to their limited size.
Accordingly, there is a need to enhance the capacity of a tandem shale shaker during high mud volume operations without increasing their xe2x80x9cfoot printxe2x80x9d, i.e., the area of floor space required by the base of the tandem shale shaker.
Accordingly, the present invention provides a shale shaker having at least a base, at least one upper shaker screen, at least one lower shaker screen, a basket, at least one distribution conduit, a flowback pan, a flow director, and a distribution apparatus. The basket is mounted movably with respect to and on the base. The at least one upper and lower shaker screens are releasably mounted on the basket. The at least one lower shaker screen is mounted at a level below the level of the at least one upper shaker screen and underlies the at least one upper shaker screen. The flowback pan is disposed between the at least one upper shaker screen and the at least one lower shaker screen and overlies at least a portion of the at least one lower shaker screen. This prevents at least a portion of the screened material falling through the at least one upper shaker screen from contacting the at least one lower shaker screen. The flowback pan directs such screened material to the at least one distribution conduit. The flow director selectively directs screened material flowing through the at least one distribution conduit (a) to the at least one lower shaker screen or (b) away from the at least one lower shaker screen. To operate the tandem shale shaker in parallel rather than in series, the distribution apparatus is set to selectively direct unscreened material to the at least one lower shaker screen and the flow director is positioned to direct the screened material from the at least one upper shaker screen away from the at least one lower shaker screen, for example, to a sump. The shale shaker preferably has an apparatus for vibrating the basket. The apparatus for vibrating the basket is interconnected with the basket.